The various applications for cameras, such as still and motion picture video cameras, continue to proliferate as technological improvements pave the way for ever-increasing uses. Various technological advances have enabled camera designers to continually reduce the size of the camera while maintaining or increasing the resolution at or beyond the resolution provided by many larger, more expensive cameras. The reduction in size of the cameras has consequently lead to several applications in which cameras are installed in one location and operated remotely from another location. Alternatively, cameras may also may be installed and configured to operate autonomously. Such applications often require the camera to track an object moving relative to the camera so that the object remains substantially centered within the field of view of the camera. As the object moves across the field of view, an electronic controller senses displacement of the object from the center of the field of view and generates control commands to displace the camera to maintain the object in proximity to the center of the field of view.
Numerous applications exist which could desirably capitalize upon such functionality. Cameras having such functionality are often employed at sporting or news events to track objects which are difficult for operator-controlled cameras to track smoothly. For example, blimps having cameras are often employed at golf events to track the flight of a golf ball which travel up to and beyond 300 yards when struck during a tee shot. Both the camera and the golf ball may be moving, further complicating maintaining the golf ball in the center of the field of view of the camera.
In other applications, such as defense and military applications, reconnaissance craft or projectiles may include cameras to track and photograph selected objects. Both the reconnaissance craft or projectile and the object may be traveling at rather high speeds and severely maneuvering, complicating maintaining the object within the center of the field of view of the camera. The challenge is to isolate the camera from the motion of the vehicle when continuing to point stably at the target.
Typically, the camera is mounted to the reconnaissance craft or projectile so that the camera case or platform is either rigidly or displaceably mounted to the body of the projectile. If the camera case or platform is rigidly mounted to the body of the projectile, portions of the camera optics are suspended within the case or platform to provide at least two degrees of freedom. If the camera platform is displaceably mounted to the body of the projectile, such as with gimbles, the camera platform moves in at least two degrees of freedom. In order to stabilize the camera and to provide a reference for target motion, a gyroscope is attached to the camera.
There exists several possible arrangements for isolating the camera from the motion of the projectile body. These arrangements include passive stabilization where the angular momentum of a large gyroscope physically stabilizes the platform and active stabilization where a small gyroscope or other device is used to measure inertial stabilization providing feedback to a stabilization loop. Such control arrangements present many difficulties to the control systems for controlling the camera to maintain the object within the center of the field of view of the camera. Either or both the object and the projectile may be moving at substantial rates of speed which require high bandwidth control in order to maintain the object within the center of the field of view. In addition, projectiles typically experience substantial vibration which may be translated to the camera and often requires filtering from the control algorithms for the camera in order to distinguish between movement of the object and vibration transferred through the body of the projectile.
In a typical camera control system, the camera controller inspects the image output by the camera, and a tracker determines an offset of the object with respect to the center of the field of view. This provides the position of the object relative to the axis of the platform or camera and defines the preferred displacement of the platform or camera in order to move the object back into the center of the field of view. In control terms, the offset is input into a tracking loop filter which generates commands in the form of a rate to displace the platform as needed. The rate includes a direction and speed for displacing the camera. The tracking loop typically operates at the same rate as the camera frame rate.
As stated above, the projectile may experience significant vibration which causes apparent displacement of the object from the center of the field of view of the camera. Because vibrations often occur continuously and vary, active stabilization systems include a stabilization loop which operates at a much higher rate than the camera frame rate. The stabilization loop typically receives feedback from a reference gyroscope attached to the camera platform. The gyroscope includes sensing mechanisms which measure the position of the gyroscope relative to the gyroscope case. The controller then generates commands for applying torque to the gyroscope at a particular rate in order to maintain the object within the center of the field of view of the camera.
More specifically, existing systems employ various approaches for maintaining objects within the center of the field of view of the camera and providing a stable platform for the camera. One such system is known as the gyroscope system. This system employs mechanical gyroscopic stabilization for the camera platform. Rather than using a small or reference gyroscope to measure and correct for disturbances, the camera platform itself is rigidly attached to the case of a large gyroscope so that the platform physically resists disturbances. When the effect of the large gyroscope does not overcome the disturbances, the tracking loop portion of the controller generates control commands to the gyroscope to displace the camera platform so that the object returns to the center of the field of view. The gyroscope system does not have a stabilization loop.
Because the tracking loop has only a single loop, the gyroscope is simple and accommodates a high bandwidth, but these benefits are traded-off against weight, power, and platform disturbance considerations. In order to isolate the gyroscope from platform disturbances, the angular momentum of the gyroscope is increased by increasing the spin rate or mass. Increasing the angular momentum, however, requires a corresponding increase in the torque required to displace the camera or platform in order to follow the object moving relative to the platform. Increased torque requires a corresponding increase in power to the torquer, the apparatus for displacing the camera platform. In addition, the platform disturbances occurring in the gyroscope couple missile body motion, such as spring torques, inertial coupling for roll about the field of view (FOV) axis, mass and balance, friction of the platform, and other disturbances, into the tracking loop. The gyroscope system does not completely satisfy the needs of systems requiring high stability and high accuracy LOS rate estimates, particularly where the missile body undergoes severe maneuvers.
In an effort to improve upon the gyroscope, designers turned to a rate platform approach. The rate platform approach does not rely on gyroscopic momentum to maintain the stability of the camera platform. Stability is maintained by sensing the camera or platform rate, comparing the sense rate to the desired rate, and applying a torque to minimize any difference between the sensed and the desired rate. Because the rate platform approach does not require a large gyroscope to maintain stability of the platform, no large angular momentum must be overcome, and the torquer power requirements for displacing the camera platform significantly decreases. The control system for the rate platform approach includes a tracking loop and a stabilization loop. The tracking loop operates at the camera frame update rate in order to determine the desired rate of platform motion. The stabilization loop operates at a much higher update rate and controls the actual rate of platform motion.
Rate platform control approaches, while addressing many deficiencies presented by the gyroscope, also offer various tradeoffs. Because the stabilization control loop is nested within the tracking control loop, the rate platform sacrifices some of the gyroscope bandwidth. Further, platform disturbances are integrated into the control loop twice in the rate platform approach, while platform disturbances are only integrated once into the control loop for the gyroscope. A double integration occurs because the gyroscope does not mechanically stabilize the platform, so that platform disturbances in the form of torques produce angular accelerations rather than angular rates. But the disturbances are measured by the platform rate sensor and cancelled by the stabilization loop. If the rate sensor disturbances are less than the disturbances to the platform, the rate platform approach produces sufficient improvement for a given weight and power.
The typical rate measuring device for the rate platform approach is a small gyroscope. Most platform disturbances do not affect the gyroscope. For example, spring torques that affect the platform do not directly affect the gyroscope because the cables and tubing that generate such disturbances are not attached directly to the gyroscope. The gyroscope simply measures the resultant effect of such disturbances. The effect manifests itself only in a second order coupling through measurement errors.
In initial rate platform implementations, the gyroscope was retained by a spring, and deflection of the spring indicated the case angle, i.e., the angle between the gyroscope axis and an axis of the container of the gyroscope. The case angle indicated the torque applied to the gyroscope. Thus, the stabilization loop was a first order, proportional control loop based on the torque applied to the gyroscope. More recently, the spring attached to the gyroscope has been replaced by an active control loop which measures the gyroscope case angle then determines the torque applied to the gyroscope. Thus, the torque to be applied to the gyroscope determines the rate at which the gyroscope is moving. When the gyroscope moves in order to follow the platform, this provides a measure of the inertial platform rate.
One drawback of the rate platform approach is that it requires three nested loops: (1) an innermost loop displacing the gyroscope to follow the platform, (2) a middle loop which determines the platform rate based on the torque required to follow the platform, and (3) an outermost loop for generating the desired platform motion based on the LOS to the target. The three nested loops limit the bandwidth of the rate platform approach and also require an extra differentiation between the platform disturbances and the feedback measurement, thereby further increasing the effect of noise. Thus, when disturbances displace the platform, the disturbance is sensed as a misalignment between the gyroscope and the platform in the form of case angle. The gyroscope is then displaced to correct this misalignment. The rate commanded to the gyroscope is sensed as a measured platform rate which differs from the commanded platform rate. A torque is then applied to the platform in order to eliminate the difference between the measured and the commanded rate. Because this control loop requires time to process, residual disturbances are fed back into the tracking loop and consequently require correction. This approach is generally considered superior to the gyroscope in many applications because it eliminates the large angular momentum and resultant torquer power required to displace the platform.
A further improvement to the rate platform approach recognizes that the quantification of platform motion is actually the rate command provided to the gyroscope. This approach is described as a forward loop implementation. The forward loop implementation controls the gyroscope directly from the tracking loop and uses the stabilization loop to drive the platform to follow the gyroscope. This eliminates high frequency gyroscopic input and reduces noise because the gyroscope control is removed from the high update rate stabilization loop and moved to the lower rate tracking loop.
The forward loop approach provides varied benefits. First, the three nested loops of the rate platform approach are reduced to two, resulting in a bandwidth increase. Second, because stabilization occurs in accordance with the gyroscope case angle rather than inferred rate measurement, a derivative step is eliminated from the feedback path. This provides both increased bandwidth and reduced noise.
In the forward loop approach, the control loop senses platform disturbances initially as changes in the gyroscope case angle. The stabilization loop corrects this directly by displacing the platform. However, unlike the rate platform approach, disturbances do not directly produce commands to the torquers for displacing the gyroscope. Some indirect coupling does occur because platform motions alter the input to the tracking loop. These residual disturbances must be first sensed and then corrected through the track loop. In order to limit this effect, the gain for the tracking loop is often reduced. Further, while the mechanical coupling of the body rate through the platform into the gyro is essentially negligible, the mechanical coupling still impacts the tracking loop estimates because portions of the tracking loop estimates feed back into the tracking loop.
The above discussed approaches each include one salient feature which also limits the ultimate performance of such systems. In each system, the platform pointing error, the difference between the target LOS and the present platform orientation, drives the track loop. Such a configuration couples body disturbances into the track loop, thereby limiting the overall effectiveness of each control approach.
Thus, it is an object of the present invention to provide a method and apparatus for enabling a camera to track an object moving relative to the camera using a gyroscopic-referenced tracking approach which is independent of platform motion.
It is a further object of the present invention to provide a method and apparatus for enabling a camera to automatically track an object moving relative to the camera by aligning the gyroscope with the object and adjusting the platform to be aligned with the gyroscope.
It is yet a further object of the present invention to provide a method and apparatus for enabling a camera to automatically track an object moving relative to the camera by providing a control system having a tracking loop and a stabilization loop, where the tracking loop displaces a gyroscope to point at the target and a stabilization loop displaces a platform to align with the gyroscope.